There are a large number of applications for motors which require variable speed such as the operation of electric tools, load lifting, fans, pumps, etc. Usually a mechanical device such as a gear box or belt is used to drive variable speed loads. Although induction motors are widely used in industry because of their ruggedness, high efficiency and low cost, their speed depends on the number of poles and the line frequency. Devices for changing the line frequency are not generally available and the number of poles is a fixed parameter.
Electric motors are ordinarily selected for specific applications compatible with speed and load requirements. In the case of induction motors, speed is directly related to the frequency of the line source. Such motors designed for use in North America have a rated frequency of 60 cycles. Increasing the frequency above the rated frequency will increase the speed but to maintain torque above the rated frequency of 60 cycles it is necessary to increase the voltage so as to keep the current/frequency ratio constant. However, the insulation of the windings will limit the voltage that can be applied so that a decreasing torque will result with increasing frequency beyond 60 cycles.
In normal operation, a motor works efficiently when it turns with a full load corresponding with its characteristics. However, a lightly loaded motor is relatively inefficient and has a corresponding poor power factor. Thus, it is desirable to reduce voltage whenever a motor is lightly loaded. Power consumption of such motors could be improved by providing a variable speed capability. Presently, variable speed is accomplished using belts or gears, assemblies which are expensive and cumbersome.
Currently, DC motors are another option for providing variable speeds. Such motors require batteries or large transformers if operated off of line voltage, thus creating an environmental problem upon disposal.
Accordingly, it is an object of the present invention to provide a simple, low cost method of varying the frequency of power used to drive induction motors.